The structure of Cretaceous oceanic crust of the NW Pacific: Constraints on processes at fast spreading centers

A 725 km long transect along a flow line in the NW Pacific provides new ima ges of the internal structure of Cretaceous oceanic crust formed at fast sp reading rates. Reflections from the Moho transition zone document changes i n crustal travel time along the profile. variations with a wavelength of 10 0-150 km may representing fluctuations in the magma supply with a 2-3 m.y. periodicity; to the south the crust may be several kilometers thicker than "normal" in places, possibly resulting from increased magma supply due to t he distal effects of the Shatsky hotspot. At about the depth expected for t he boundary between seismic layers 2 and 3, sub-horizontal reflections are imaged over much of the profile; the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of similar to 0.1, im plying a sharp velocity and density discontinuity. Northward (ridgeward) di pping reflections in the lower crust are imaged over much of the profile, p articularly toward the north (away from the region possibly affected by the hotspot). These reflections stop at the Moho reflection but die out perhap s 1 s beneath top basement; they probably represent lithological layering a s predicted by models of ductile flow accompanying passive upwelling during plate separation, although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center. The discrete character and, in places, regular spacing (every similar to 8 km) of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0.1-0.2 m.y.